This invention relates to a multi-terminal power line protection system for protecting the power transmission lines constituting a multi-terminal power network.
In general, faults, such as short circuits or ground faults, occurring in a power network are eliminated from the power system as in the following manner. Protective relays are provided for the respective system components such as transmission lines, transformers, buses, generators, etc. which constitute the power system. When a fault occurs in a certain area of a transmission line of the power network, a protective relay covering the area of fault operates to trip a circuit breaker for the area and thereby to isolate the fault area from the rest of the sound network. The most prevalent example of such a protective relay system nowadays is a digitally operated protective relay system in which an algorithm is installed that digitalizes the current and voltage data obtained from the power system and determines system faults through the processing of the current and the voltage data by software.
As a role of the protective relay system for the power network, it is desired to accurately detect a fault in an area of protection which it is supposed to cover, to narrow down the region affected by the fault to the minimum extent possible, and to isolate the fault from the power system rapidly. Therefore, it is ideally desired that the protective relay supposed to operate first in the event of fault should operate as swiftly as possible in response to the fault occurring in the area of protection but not operate in response to a fault occurring outside of the area of protection.
Today, among protective relays used for the protection of power transmission lines that employ a calculation principle for detecting faults and determining the section in which the fault is occurring, most prevalent relays are a distance relay of impedance type which determines whether a fault is within or without the area of protection by calculating the impedance between the relay and the point of the fault on the basis of the data of voltage at a single terminal and of current flowing through the entire transmission line of interest; and a current-driven differential type protective relay which obtains data on currents at all the terminals of the transmission line through sampling and performs necessary arithmetic operations on the basis of Kirchhoffs current law.
Especially, more prevalent is the latter current-driven type differential protective relay which can distinguish a fault occurring within the area of protection from a fault occurring outside of the area of protection. JP-A-7-336874 is known, for example, as an application of such current-driven type differential protective relay to the protection of a multi-terminal power transmission line.